Magnetic Force Microscopy (MFM) and Magnetic Resonance Force Microscopy (MRFM) use mechanical resonators to discern material features approaching the atomic dimension level. In MFM, a ferromagnetic probe is scanned over a material under test and maps stray magnetic fields close to the surface. Resolution is routinely better than 50 nm, much better than the best optical techniques. Sensitivity is sufficient to image structures at the submicron level. In MRFM, the ferromagnetic tip is scanned over a material under test while an appropriately modulated radio frequency (RF) field is applied by a small coil and magnetic resonance signal from the spins (both nuclear and electron) contained in the sample is detected. Thus, unlike the other scanning probe techniques, MRFM provides a material specific subsurface information about a sample under the study.
As stated, both MFM and MRFM use ferromagnetic tips at the end of a weak double-clamped beam or cantilever that are used as probes to determine surface and other properties of the material under test. Normally, these probes are magnetically sensitized through sputter coating with a ferromagnetic material. In this sputtering process, the entire cantilever and tip are coated with ferromagnetic material. Because of this, the behavior of these cantilevers is problematic. This results from the fact that spatial resolution of the intended measurement is defined to the first order by the characteristic size of the probe magnet. Effective reduction of the size of this probe magnet can be achieved by using a conical or pyramidal tip at the end of the cantilever. However, even with this type of tip, if the total cantilever and tip are made magnetic, the spatial resolution suffers.
Several problems are encountered when the cantilever and tip are coated with ferromagnetic material. When operating at low temperatures and in external RF fields, these problems include: (a) differential thermal contraction between the native material of the cantilever and the coating layer magnetic material, which results in stressing and bending of the cantilever; (b) the presence of the layer of metallic ferromagnetic material results in parasitic heating of a cantilever in an external RF field; and (c) excessive magnetic material on the cantilever results in significant resonant frequency shifts in external magnetic fields. Therefore, a significant need exists for limiting the ferromagnetic layer to the tip of the cantilever in order to avoid these problems.
It is therefore an object of the present invention to provide a cantilever and tip that provides accurate information on a material under test.
It is another object of the present invention to provide a method for assuring that magnetic material is only on the tip of a cantilever used in MFM and MRFM.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method of producing a layer of thin ferromagnetic coating on the tip of a cantilever used as a mechanical resonator in magnetic resonance force microscopy and magnetic force microscopy comprises the steps of spinning the cantilever in the presence of a photoresist such that the photoresist spreads along the cantilever but not onto the incorporated tip; sputtering a ferromagnetic layer onto the cantilever and the incorporated tip; removing the ferromagnetic layer from the cantilever, but not from the incorporated tip.
In a further aspect of the present invention, and in accordance with its principles and purposes, a mechanical resonator for use in magnetic force microscopy and in magnetic resonance force microscopy comprises a cantilever with an incorporated tip, the incorporated tip being coated with a layer of ferromagnetic material.